The invention relates to holey fibres and to a method of fabricating holey fibres and holey fibre preforms.
A holey fibre is an optical fibre whose optical confinement mechanism and properties are affected by an array of air holes defined by cavities that run lengthwise down the fibre. Light can be guided within holey fibres by two distinct mechanisms. First, with periodic arrangements of air holes, guidance can be obtained through photonic band gap effects [1]. Second, guidance can be obtained from volume average refractive index effects. This second guidance mechanism does not rely on periodicity of the air holes [2].
Generally, a holey fibre has a solid core (FIG. 1A of the accompanying drawings) or a hollow core (FIG. 1B of the accompanying drawings) surrounded by a holey cladding region. The holey fibres illustrated have a hole structure characterised by a hole diameter, d, and an interhole spacing, i.e. pitch, Λ.
A holey fibre structure is fabricated by stacking tubular capillaries in a hexagonal close packed array within a larger tube that forms an outer jacket or casing containing the capillaries. To form a solid core holey fibre as in FIG. 1A, one of the tubular capillaries is removed from the stack and replaced with a solid rod of the same outer dimensions. To form a hollow core holey fibre as in FIG. 1B, a number of capillaries in the centre of the stack are removed. The fibre stack is then drawn into a preform by a caning procedure and then placed in a fibre drawing tower and drawn into fibre. The finished holey fibre structure is then characterised by an inner core (solid or hollow) surrounded by a holey cladding. Fabrication of holey fibres is discussed further in the literature [3][4].
To realise holey fibres for many applications, it is desirable to fabricate a holey fibre with relatively small feature sizes, such as interhole spacing, i.e. pitch, Λ˜1–2 microns. Fibres with such small hole feature sizes have a number of interesting and unique properties such as anomalous dispersion at short wavelength, high optical nonlinearity and the possibility for large evanescent fields in air.
To satisfy the desire for small pitch, it is necessary to construct a preform structure with relatively small capillaries. Because of the small size of the capillaries, several hundred capillary elements are needed to provide a structure which is large enough to handle conveniently during the fabrication stages of preform caning and fibre drawing. Moreover, to be practical, the fabricated fibre needs to have an outer diameter of about 80 microns or more. However, the large number of small capillaries required to fulfil these requirements presents difficulties in the fabrication and also results in a weak fibre.
An improvement is to stack the capillaries within an outer jacket which has a relatively thick wall, as shown by FIG. 2 of the accompanying drawings which shows a thick wall silica outer jacket 1 defining an inner cylindrical space in which is placed two rings of silica cladding capillaries 2 which are arranged concentrically about a centrally placed solid silica core 4. In the illustrated example, the inner space of the outer jacket 1 is additionally sleeved by a vycore tube 3. The dimensions included on the top of the figure are exemplary preform dimensions in millimetres, whereas the dimensions at the bottom of the figure are target fibre dimensions in microns. Use of a thick wall outer jacket has the advantage of allowing the number of capillaries required to be greatly reduced.
The thick wall outer jacket approach has been demonstrated by other groups. However, in the experience of the present inventors at least, it has proved difficult to reliably and controllably retain small hole features during the fibre pulling stage of the fabrication process when using such thick walled outer jacket structures. It is believed that this problem is attributable to the relatively small ratio of air to glass in the thick-walled structure, and to the relatively large thermal mass of the glass of the outer jacket as the preform is melted in the drawing tower furnace during the fibre drawing process.
It is therefore an aim of the invention to provide an improved method for fabricating holey fibres with relatively small feature sizes.